1. Field of Invention
The invention relates to a control system for controlling a power supply of a vehicle, especially for controlling a plurality of power supply portions of a vehicle, and a control method thereof.
2. Description of Related Art
In a vehicle, generally, a power storage device and power generator are provided and together serve as a power supply for supplying power to a variety of electric devices of the vehicle. The power generator is normally driven by an internal combustion engine as a driving power source of the vehicle. Recently, however, it has been strongly demanded to improve the fuel economy mainly for the purpose of reducing exhaust gas emitted from the vehicle. In response to such a demand, it is attempted to regenerate power from kinetic energy of the vehicle and store the regenerated power in the power storage device.
One of typical examples of such a vehicle is a so-called “hybrid vehicle” which includes an internal combustion engine and motor-generator as its driving power sources. A vehicle of this type is adapted to start moving (take off) only using the motor-generator in a certain operating state thereof, and it is therefore required that a power supply of the vehicle is capable of outputting a large amount of power in a moment. For this reason, a capacitor that stores power in the form of static electricity is used as a power supply for the motor-generator and as a power storage device for storing the power regenerated by the motor-generator.
Moreover, there has recently been developed a vehicle which performs a so-called economy running operation. In this operation, idling of the engine is stopped when the vehicle pulls over (temporarily stops) and the engine is automatically restarted when the conditions for defining the stopped state of the vehicle become unsatisfied. Thus, the total time for the engine idling is reduced, which leads to a reduction in the amount of exhaust gas emitted from the vehicle and to an improvement of the fuel economy.
In such “an economy-running vehicle”, a large amount of power is required for restarting the engine immediately. Therefore, a capacitor capable of discharging a large amount of power in a moment is used as a power supply for an engine starter instead of a conventional battery, and also a power regenerator for generating power for charging the capacitor may be provided together with the capacitor.
As disclosed in Japanese Laid-open Patent Publication No. 2000-156919, it is known that a capacitor that has a high power storage voltage and is capable of discharging a large amount of power in a short time is arranged such that the power from by the capacitor is supplied to a battery to charge it. Such an arrangement makes it less necessary to drive an alternator by an engine for charging the battery, and the quantity of fuel applied to the alternator is reduced, whereby the fuel economy improves.
It is true that the fuel economy improves when power regeneration is performed using the capacitor and power regenerator as described in the above publication. However, it is considered that the regenerated power is not effectively used in many cases, and therefore further improvements are obviously necessary for enabling a more effective use of the regenerated power.
In the case what the capacitor is used for charging the battery as described in the above publication, it is necessary to stop charging the battery by the alternator before the battery is fully charged, so that the battery can always receive the power supplied from the capacitor. However, since the power stored in the capacitor is obtained by performing power regeneration and is therefore not always available, the charged capacity of the battery may be short when discharging power therefrom. If the battery is always kept fully charged using the power generated by the alternator in order to avoid such a power shortage, it may happen that the power from the capacitor can not be received by the battery, the battery is excessively charged due to increased output voltage of the capacitor, or an electric load connected to the battery wastefully consumes excessive amount of power.
By the way, when a power regenerator and capacitor are provided in a vehicle as described above, it means that the vehicle has two power supply portions; a first power supply portion constituted by an alternator or by an alternator and battery, and second power supply portion constituted by a power regenerator or by a power regenerator and capacitor. Considering the fact that the second power supply portion for power regeneration can be effectively used for improving the fuel economy but can hardly be “a stable power supply”, it is preferable that the power regenerated by the second power supply portion is first used and the power generated by the alternator or stored in the battery is used when the regenerated power becomes insufficient.
For doing this, it is necessary to switch from one of the power supplies to the other while supplying power to the electric load. In order to enable such a switching between the power supplies, for example, the alternator is adapted to start supplying power when the power that is currently being supplied from the capacitor to a certain electric load reduces. In this case, almost no torque is applied to the alternator when power is being supplied from the capacitor, and a certain level of torque is applied to the alternator when the alternator is substantially connected to the electric load to start generating power. At this time, if the electric load is large, large torque is suddenly applied to the alternator, which may cause a slippage of a belt connecting an internal combustion engine and the alternator, and such a belt slippage may sometime produce a noise and may reduce the durability of the belt.
If power is gradually generated by the alternator to avoid such a problem, the output voltage of the alternator may still be low when the power supply from the capacitor is cut off. Such a reduction in the power supplied to the electric load may cause a momentary change in its operating state, such as a momentary reduction in the luminance of a light, which may lead to an unpleasant feeling of passengers.